PROJECT SUMMARY (See instnjctions): The overall goal of the research program is to understand flexibility in motor skill acquisition-how infants, children, and adults learn to select appropriate actions, modify ongoing actions, and construct new actions in accordance with thecurrent constraints on action. Flexibility is imperative for adaptive, functional action because local conditions are continually changing. A primary source of variability is the body. Changes in body dimensions and abilities are particularly dramatic during infancy, but throughout the life span, both permanent and temporary alterations to the body change the biomechanical constraints on action. The proposed research aims to: (1) Compareflexibilityin in^nt motor skill acquisition to older children, young adults, and elderly adults based on changes in body constraints; (2) Characterize how body constraints affect the perceptual information obtained during spontaneous visual exploration; and (3) Investigate effects of body constraints in specialized perception-action systems. The studies will use apparatuses and procedures (ascending/descending adjustable cliff, reaching and locomoting through apertures, dynamic reaching in the context of ongoing movement, brachiating over monkey bars) and technologies (head- mounted eye-tracking, video tracking, computerized behavioral coding, hfiotlon tracking) developed in the previous project period to track changes inflexibilityduring leaming and development in infancy and across the life span.